1. Field of the Invention
The present invention relates to a buck circuit; in particular, to draw leakage current of a low dropout voltage regulator timely.
2. Description of Related Art
Recently, because of characteristics of the enhancement for the conversion efficiency, small volume and low noise, the low dropout voltage regulator becomes a mainstream of buck circuit with low power and regulator circuit. In the wide range of the portable system and the electronic product related to the communication that the battery provide the power, the low dropout voltage regulator is widely used.
The low dropout voltage regulator may be adapted to multiple electronic equipment, such as notebook computercell phonepersonal digital assistant but is not limited thereto, and the low dropout voltage regulator may provide stable output voltage for the load of the electronic equipment.
Referring to FIG. 1, FIG. 1 shows a circuit diagram of the conventional low dropout voltage regulator. The low dropout voltage regulator includes a comparator OP′, P-type transistor MP′ and the resistances R1R2. The negative input end of the comparator OP′ receives the feedback voltage VF′ and the output end of the comparator OP′ outputs the voltage V1′. A gate of the P-type transistor MP′ receives the voltage V1′, a source of the P-type transistor MP′ receives the input voltage VIN′, and a drain of the P-type transistor MP′ output the output voltage VOUT′. One end of the resistance R1 is electrically connected to the source of the P-type transistor so as to receive the output voltage VOUT′. Another one end of the resistance R1 is electrically connected to one end of the resistance R2 so as to output the feedback voltage VF. Another one end of the resistance R2 is electrically connected to the ground voltage GND.
If the conventional low dropout voltage regulator 100 ideally works under normal operation, the low dropout voltage regulator 100 may provide stable output voltage VOUT′ through the negative feedback mechanism within itself. More specifically, in normal operation situation, the output voltage VOUT′ is determined by the reference voltage VREF′. When the negative feedback mechanism exists efficiently, the feedback voltage VF′ is equal to the reference voltage VREF′, i.e. VOUT′=(1+R1/R2)VREF′.
However, in non-ideal situation, when the input voltage VIN′ is a voltage outside the normal operation range of the low dropout voltage regulator 100, or the low dropout voltage regulator 100 works in high temperature or in fast process corner, the P-type transistor MP′ may generate a leakage current. Meanwhile, the current I1′ is the leakage current. Next, when the leakage current flow through the resistances R1R2, the feedback voltage VF′ and the output voltage VOUT′ may be further elevated. Accordingly, the negative feedback mechanism within the low dropout voltage regulator 100 is to be destroyed, and then the output voltage VOUT′ may approach the input voltage VIN, and thus cause damage of the load which receives the output voltage VOUT′.